ABSTRACT
The need to increase the number of channels in a limited radio spectrum while covering a large area in a mobile cellular network has lead to the use of various cell design techniques. With an increased number of channels, handover calls can be dropped while moving from one cell to another in the search of a better signal or as a result of user mobility. New calls are also blocked when the traffic channels are fully occupied. However, new calls should be treated fairly and not to be entirely blocked from gaining access to the network as a result of the lack of free channels when the handover requests are absolutely comfortable. In this project, the challenges associated with blocked new calls and dropped handover calls, which are the most important determinants of Quality of Service (QoS) in a cellular network have been carefully examined. First, service optimization is achieved by employing a reservation scheme and a retrial queueing scheme. Secondly, an analytical model that employs the phase merging technique has been formulated. A two dimensional continuous time markov chain (CTMC) was also introduced to provide a complete description of the system states and their transitions. The simulation results show that the reservation scheme with retrial has improved the system performance by minimizing to a large extent the blocking probability and the handover dropping probability. These results have also been recommended to the network operator for service optimization of the cellular network even under high load conditions.
TABLE OF CONTENTS
APPROVAL PAGE – – – – – – – – i
CERTIFICATION – – – – – – – ii
DEDICATION — – – – – – – – iii
ACKNOWLEDGEMENT – – – – – – – iv
ABSTRACT – – – – – – – – – v
TABLE OF CONTENTS – – – – – – – vi
LIST OF FIGURES – – – – – – – – viii
LIST OF TABLES – – – – – – – – x
LIST OF ACRONYMS – – – – – – – xi
CHAPTER ONE
INTRODUCTION – – – – – – – – 1
1.0 Background of study – – – – – – – 1
1.1 Problem statement – – – – – – – – 2
1.2 Aim and objectives of the research – – – – – – 3
1.3 Scope of the research – – – – – – – 3
1.4 Research Methodology – – – – – – – 4
1.5 Thesis Outline – – – – – – – – 5
CHAPTER TWO
LITERATURE REVIEW – – – – – – – 6
2.0 Introduction – – – – – – – – 6
2.1 GSM physical architecture – – – – – – – 6
2.2 The Concept of Cellular Telecommunication – – – – 10
2.3 The Principles of Cellular Communication – – – – – 12
2.4 Cellular Design Techniques – – – – – – 17
2.5 Handover – – – – – – – – – 22
2.6 Classification of Handovers – – – – – – 24
2.7 Reasons for Handover – – – – – – – 25
2.8 Types of Handover – – – – – – – 26
2.9 Handover Initiation – – – – – – – 31
2.10 Handover Decision – – – – – – – – 32
2.11 Call Handover Control Procedures – – – – – 33
2.12 Popular Handover Schemes – – – – – – 39
2.13 Related Works – – – – – – – – 44
CHAPTER THREE
MODEL DEVELOPMENT – – – – – – – – 47
3.0 Introduction – – – – – – – – 47
3.1 Physical Model – – – – – – – – 47
3.2 Analytical Model – – – – – – – – 49
3.3 Model Validation – – – – – – – – 55
CHAPTER FOUR
SIMULATION AND RESULT EVALUATION – – – – – 60
4.0 Introduction – – – – – – – – 60
4.1 Blocking probability without retrial versus total traffic intensity (g=0): – 57
4.2 Blocking probability with retrial versus total traffic intensity (g=3): – 58
4.3 Handover dropping probability versus total traffic intensity: – – 59
4.4 Handover dropping probability versus number of channels: – – – 61
4.5 Comparison of the different responses – – – – – 62
CHAPTER FIVE
CONCLUSION AND RECOMMENDATION – – – – 64
5.0 Conclusion – – – – – – – – 64
5.1 Recommendation – – – – – – – – 64
REFERENCES – – – – – – – – 66
CHAPTER ONE
INTRODUCTION
1.0 BACKGROUND OF STUDY
The mobile cellular system is affected by an exponential growth of subscribers. It is therefore imperative for the communication industry to provide better coverage and improved network capacity as well as an effective call control procedure to optimize network usage [1, 2]. As a result of the limited frequency spectrum and a limited number of channels per coverage area, the cellular concept was introduced to solve the problem of spectral congestion and user capacity [1]. However, this concept has over time faced challenges due to cell overlap and an increased traffic volume. This has resulted to an increase in the number of calls that are dropped or blocked while a communication is in progress or while trying to gain access to the network. Thus subscribers are unsatisfied with the quality of service rendered by the network operator. The network operator’s primary goal which is to ensure that his customers are satisfied can only be achieved if standard QoS is maintained at the network [1, 3, 4 ]. In this case QoS comprise the speech quality as well as the availability and reliability of service within the coverage area. With the increase in traffic intensity, QoS depend more on the uncommitted network resources and on the teletraffic engineering of the network. Standard QoS can be maintained if the number of dropped handover calls is minimized or insignificant. Therefore the network must have a very low handover call dropping probability. On the other hand, the new calls arriving at the network may be blocked if there are no free/common network resources available at their time of arrival. Therefore the new call blocking probability must also be made to be as small as possible.
The handover dropping probability and the new call blocking probability determine the QoS of a cellular network and can be defined as the performance metrics of the network. In this project, these performance metrics have been investigated on an isolated cell in a cellular mobile network with two types of calls, the new calls and the handover calls. From the investigation, an analytical model was formulated. The model was formulated by employing the guard channel scheme and the retrial queue into the network for handover and new calls respectively.
In a GSM based cellular system, those calls that face blocking may automatically retry for service for specified number of times defined as a call retrial. Therefore a retrial pattern may consist of a maximum number of attempts that are separated in time by random time intervals. In the GSM standard, automatic retrials are permitted for both dropped handover calls and new calls. The number of retrials and the inter-retrial times are controlled by parameters that are relayed regularly through the broadcast control channels [5].
Although there is vast literature on this subject, the analysis of the cellular network through the use of different modelling tools – the retrial queue, guard channels, continuous time markov chain (CTMC) and the phase merging algorithm (PMA) has provided a method of optimizing the QoS parameters of the network while varying the traffic intensities.
1.1 PROBLEM STATEMENT
In a mobile cellular network, it is the expectation that all incoming calls or requests get served immediately [1]. This is not usually the case since incoming calls are sometimes blocked due to the lack of free channels. To increase the number of channels needed for effective call completion, cell design techniques such as sectoring, cell splitting, zone microcell techniques are employed [1, 6]. These techniques are used to increase the number of available cells in a given network area. This increase causes cell sizes to shrink and cells tend to overlap [6, 7]. Therefore, ongoing calls may have the need to transfer from a serving cell to an adjacent cell as a result of poor signal reception or in some cases as a result of user mobility. In that case, the ongoing call is technically handed over to an adjacent cell with a better signal strength. In the case where the adjacent cell with a better signal reception has no free radio channel to support the ongoing call, the call is terminated forcefully. The forced termination of an ongoing call causes a reduction in the QoS standard for the network.
The new call blocking probability and the handover dropping probability must be carefully monitored in order to prevent customer dissatisfaction, cell congestion, and waste of limited resources e.t.c. Both metrics have a level of importance to the network and so attention to any one of the metric should not be at the expense of the other. This implies that new calls are as important as the handover calls, therefore both type of calls should be treated fairly. It is also required that the performance metrics be kept at an optimal level in order to limit the effect of blocked and dropped calls on the network.
1.2 AIM AND OBJECTIVES OF THE RESEARCH
The aim of this project is to arm the network operator with a tool with which to optimally allocate resources to handover calls while considering the new calls. The project also aims to provide customer satisfaction by reducing the number of calls that are blocked or forcefully terminated in a mobile cellular network, even with increased traffic volume.
1.3 SCOPE OF THE RESEARCH
There are various causes of active call drops apart from handovers and inadequate channel resources; they include battery failure, propagation conditions and irregular user behaviour [8]. This research is limited to only the effect of handovers on active calls. The research also covers how the limited channel resources affect all incoming requests into the network. A cell is studied in isolation from the rest of the system. The interaction with adjacent cells is taken into account by means of the incoming handover requests. The cells are considered to be identical and to have the same traffic parameters. A single traffic system such as voice traffic with each cell having only a finite number of channels is applied.
1.4 RESEARCH METHODOLOGY
First a detailed study of the mobile cellular network was carried out with the Global System for Mobile (GSM) network as a case study. The essence of the study was to have a good knowledge of the network architecture and implementation, its principles and techniques, the problems associated with the network implementation, the methods that could be used to tackle such problems, their merits and demerits. Various methods proposed by other researches were also studied to find the best tool needed to tackle the problems of the cellular network.
The tools that were considered are the conventional guard channel scheme (simply referred to as the guard channel scheme) and the retrial queue. The guard channel scheme reserves a number of channels exclusively for handover calls, thereby reducing the number of calls that are forcefully terminated during a communication [2, 7, 8]. The guard channel scheme has other antecedents like the Fractional Guard Channel (FGC), Limited Fractional Guard Channel (LFGC), Uniform Fractional Guard Channel (UFGC) schemes and so on [9]. The conventional guard channel scheme was selected because it reduces system complexity and requires fewer control parameters. It has also been shown that few guard channels are needed to reduce the volume of handover call drops [8, 10].
Another tool that was considered is the retrial queue. Usually a call after a random time may return to the network and try to get service again [11]. Therefore the retrial queue was introduced to the network to solve the problem of call reattempts. This is especially true for the new calls arrivals that find all the common network resources fully occupied and tend to retrial for service again after a random time, following a random discipline and at random intervals thus realizing a network that is close to a practical situation.
The guard channels and the retrial queue were integrated into the network and a two dimensional CTMC was developed to determine how the network responds by modelling the system states and the transitions between the states. Modelling the system states involved considering the events that change the resource states at the cell (or node) and the user behaviour with respect to call retrials. The states of the system were described by a dual variable random process in continuous time. To formulate the analytical model, a mathematical tool known as the phase merging algorithm was used to determine the steady state probability of the system. A simulation was carried out on the formulated model to determine the call blocking probability with and without guard channels, the blocking probability with guard channels and a retrial queue, the handover dropping probability. Finally, a comparison of the different responses was done to ascertain the optimal level of the blocking probability and handover dropping probability needed for improved QoS under varying traffic load conditions.
1.5 THESIS OUTLINE
This rest of the project is organized as follows;
In chapter two, an extensive literature review on GSM physical architecture, the cellular concept, cell design techniques, handover schemes and also a review of relevant works on the retrial queue and the guard channel scheme is presented. In chapter three, a model and physical representation of the system was presented. An analytical model for the approximate determination of the blocking probability and the handover dropping probability as a function of some network parameters like the number of channels, the number of guard channels and the traffic intensity was formulated and validated. In chapter four, the results of the simulation showing the various responses is presented. The results were evaluated and meaningful deductions made from them. In chapter five, conclusion on the project as well as recommendations to the network operator was made.
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